Cover unpaired surrogate escapes in the carrier corpus

[kikashy]

Goal

Add carrier-conformance cases for JSON string escapes that do not form a valid surrogate pair.

Why

The corpus mentions surrogates nowhere: grep -c surrogate conformance/manifest.json returns 0.

The Go reference runtime refuses them, and deliberately — internal/carrier/decode_test.go carries a twelve-case table for lone high surrogates, lone low surrogates, reversed pairs, one inside an array, one in a member name, and the valid pair that must still be accepted. None of that is in the corpus, so a second implementation has no case telling it this is required.

§2.1 requires rejecting malformed input rather than processing a silent prefix. An unpaired \uD800 is malformed input that many JSON parsers will happily turn into a replacement character, which is exactly the "silent prefix" shape the rule exists to prevent.

Scope

• Add fixtures under conformance/carrier/ for at least:
  • a lone high surrogate escape in a string value;
  • a lone low surrogate escape;
  • a reversed pair (low followed by high);
  • a valid surrogate pair, which MUST be accepted.
• Add matching entries to conformance/manifest.json near the other carrier cases, with layer: carrier, the appropriate expectedResult, and the diagnostic code and path the repository's own validator produces.
• Update the document-corpus count statements — a harness test derives them, so it will tell you which.
• Do not change the schema, the validator, normative text, or existing cases.

Acceptance criteria

• Each listed escape has a fixture and a manifest entry.
• The valid pair is present and expected valid, so the cases distinguish "unpaired" from "surrogate".
• Expected diagnostics match what the repository's validator actually reports; run the suite to find out rather than guessing.
• python -m unittest discover -s tests -v passes.
• Every commit is signed off (git commit -s).

Contributor learning

The carrier layer, why an escape that decodes to a replacement character is a conformance question, and how a corpus case tells another implementation what is required.